1. Field of the Invention
This invention generally concerns a laser apparatus, in particular, such a laser apparatus that has a function to measure an oscillated wavelength of a laser beam output from a laser oscillator by using as a reference a component, which has a wavelength near to the oscillated wavelength of the laser beam, selected from among components of a reference beam having spectral distribution which is already known.
2. Description of a Related Art
At an exposure process in producing a semiconductor device, a micro-fabrication of pattern to be transcribed to the resist coated on a substrate is going on, and improvement of definition of an exposure system is demanded. On this account, as a light source of the exposure system, a laser apparatus for outputting a narrow-banded laser beam attracts attention. In such a laser apparatus, a laser oscillator needs to be controlled so that an oscillated wavelength of a laser beam is measured and is kept at a desired value.
Japanese patent publication JP-A-5-95154 discloses a laser apparatus for outputting an atomic fluorine laser beam and a narrow-banded molecular fluorine laser beam and having a function to measure an oscillated wavelength of the molecular fluorine laser beam by using an oscillated wavelength of the atomic fluorine laser beam as a reference.
However, in this laser apparatus, the oscillated wavelength of the atomic fluorine laser beam is in a visible range, and therefore, is different from the oscillated wavelength of the molecular fluorine laser beam. Accordingly, a narrow-banded oscillated wavelength of the molecular fluorine laser beam can not be measured with high precision. Thus a correct reference becomes necessary to measure a wavelength of a beam having a narrow spectral distribution such as a narrow-banded laser beam.
In the other hand, Japanese patent publications JP-B2-2631553 and JP-B2-2631569 disclose an apparatus including reference light source for outputting a reference beam that contains a component having a wavelength near to an oscillated wavelength of a laser beam. The reference beam is spectrum-separated by a spectroscope. The apparatus has a function to select a component, which has a wavelength near to the oscillated wavelength of the laser beam, from among components of the reference beam output from the spectroscope.
In a chamber of the reference light source, atoms and molecules are generally enclosed which emit a beam containing many wavelength components. When such a beam is input to a spectroscope, a lot of wavelength components are provided. In the same time, there may occur a phenomenon that a wavelength component of a beam belonging to a predetermined wavelength range happen to appear in the different wavelength range from the predetermined wavelength range (that is to say, xe2x80x9can overlappingxe2x80x9d) when the beam is separated by spectrum separation means having high resolving power (for example, by an etalon or an echelle grating). The overlapping rate is increased higher if resolving power of the spectrum separation means becomes higher.
In a laser apparatus for outputting a narrow-banded laser beam, in order to measure an oscillated wavelength of the laser beam more precisely, it is necessary to separate components of a reference beam output from a reference light source by using spectrum separation means having high resolving power and to select among the separated components a component, which has a wavelength near to the oscillated wavelength of the laser beam, as a measurement reference. However, in the laser apparatus in which the reference beam output from a reference light source is separated by one spectrum separation means, a lot of wavelength components of the separated reference beam happen to focus on a detector with being very close or overlapped. Therefore, it is difficult to select a component, which has a wavelength near to an oscillated wavelength of the laser beam, among the detected components of the reference beam.
The present invention has been accomplished under these circumstances. An object of the present invention is to provide an laser apparatus having a function to easily select a component having a wavelength near to an oscillated wavelength of a laser beam among components of a reference beam output from a reference light source as a reference in measurement of an oscillated wavelength of the laser beam.
In order to solve above problems, a laser apparatus according to the first aspect of the invention comprises a laser oscillator for outputting a laser beam; a reference light source for outputting a reference beam having a spectral distribution which is already known; first spectrum separation means for spectrum-separating the laser beam output from the laser oscillator and spectrum-separating the reference beam output from the reference light source; second spectrum separation means having a higher resolving power than that of the first spectrum separation means and for spectrum-separating a part of the laser beam and a part of the reference beam emitted from the first spectrum separation means; and a detecting device for detecting a part of the laser beam and a part of the reference beam emitted from the second spectrum separation means.
Also, a laser apparatus according to the second aspect of the invention comprises a laser oscillator for emitting a laser beam; a reference light source for outputting a reference beam having a spectral distribution which is already known; first spectrum separation means for spectrum-separating the laser beam output from the laser oscillator and spectrum-separating the reference beam output from the reference light source; second spectrum separation means having a lower resolving power than that of the first spectrum separation means and for spectrum-separating a part of the laser beam and a part of the reference beam emitted from the first spectrum separation means; and a detecting device for detecting a part of the laser beam and a part of the reference beam emitted from the second spectrum separation means.
Further, a laser apparatus according to the third aspect of the invention comprises a laser oscillator for outputting a laser beam; a reference light source for outputting a reference beam having a spectral distribution which is already known; first spectrum separation means for spectrum-separating the laser beam output from the laser oscillator and spectrum-separating the reference beam output from the reference light source; second spectrum separation means having a different resolving power from that of the first spectrum separation means and for spectrum-separating a part of the laser beam and a part of the reference beam emitted from the first spectrum separation means and for emitting the separated part of the laser beam and the separated part of the reference beam toward the first spectrum separation means; and a detecting device for detecting a part of the laser beam and a part of the reference beam emitted from one of the first spectrum separation means and the second spectrum separation means after the part of the laser beam and the part of the reference beam have shuttled a predetermined times between the first spectrum separation means and the second spectrum separation means.
According to the present invention, spectrum separation means with low resolving power removes a component, which has a wavelength greatly different from an oscillated wavelength of the laser beam, from the reference beam. In contrast, spectrum separation means with high-resolution power spectrum-separates the reference beam with high resolution. Each of these two kinds of spectrum separation process should be performed for the reference beam once at least. Hereby, the outbreak of xe2x80x9can overlappingxe2x80x9d is effectively suppressed and a lot of separated wavelength components of the reference beam can be prevented from focusing with being very close or overlapped on the detector. Accordingly, a component having a wavelength near to an oscillated wavelength of the laser beam is easily selected in the reference beam and defined as a reference in measurement of an oscillated wavelength of the laser beam. Thus an oscillated wavelength of the laser beam can be measured with high precision by using the selected wavelength component as a reference.